Members of high temperature apparatuses such as moving blades and stationary blades of jet engines and gas turbines, combustors, boiler tubes and emission gas purification mufflers of vehicles are often surface-coated for improving their heat resistance and corrosion resistance.
To enhance high temperature corrosion resistance of alloy-made substrates, in general, a coating film is formed by vapor diffusion of Al, Cr, Si, or the like, or diffusion coating such as pack cementation. Such a coating film has a function of producing protective oxide scales (such as Al2O3, Cr2O3, SiO2, etc.) of Al, Cr, Si, etc., respectively.
Furthermore, an oxide film protecting the substrate is formed by an overlay coating layer of a high Cr—Ni alloy or MCrAlY (M=Ni, Co, Fe) alloy. Sometimes, the overlay coating layer may be further coated with a ceramic coating. This is generally called thermal barrier coating (TBC). The overlay coating layer is called a bond coat layer as well, and the ceramic coating layer is called a topcoat layer as well. For forming the TBC layer, thermal spraying, electron beam vapor deposition, or the like, is typically used. Layers containing Al such as MCrAlY among these overlay coating layers are called aluminum reservoir layers as well. They form protective Al oxides and thereby maintain sound conditions of coating layers and heat-resistant metal substrates under the coating layers. Under ultra high temperature environments as high as 800 to 1200° C., however, Al atoms contributing to corrosion resistance by forming protective oxide scales (Al oxides) diffuse into the part of the substrate from the coating layer, and alloy elements such as Ni, Co, Ti, Ta, Mo, S, Cr, etc. contained in the substrate diffuse into the part of the coating layer, thereby causing a change in elements themselves and their concentrations of the coating layer. As a result, the oxide scales lose the protective ability with time.
As such, under ultra high temperature environments, diffusion coating or overlay coating lose the ability of forming and maintaining an oxide film for protecting the substrate, as well as the ability of reproducing an oxide to compensate a peeled film portion with time.
Moreover, under ultrahigh temperature environments, mutual diffusion progresses between the coating layer and the substrate, and a resultant change in texture and composition of the substrate causes deterioration of mechanical characteristics of the substrate as well. As a problem of substrates such as Ni-based single crystal superalloy, Ni-based superalloy, Ni-based heat-resistant alloy, and the like, it is known that intermetallic compounds generally called topological closed packs (TCP) precipitate and degrade creep, fatigue and other mechanical properties. This phenomenon is especially prominent in Ni-based single crystal superalloys. More specifically, Al having diffused into the part of the substrate destroys the texture of the substrate (mixture of γ and γ′ phases) in case the substrate is a third generation Ni-based single crystal superalloy, for example, and thereby forms a TCP phase. This results in significantly deteriorating mechanical properties of the Ni-based single crystal superalloy having formed an alloy film on the surface (see, for example, Y. Aoki, M. Arai, M. Hosoya, S. Masaki, Y Koizumi, T. Kobayashi, Engine Rotor Application, Status and Perspective, Report of the 123rd Committee on Heat Resisting Materials and Alloys, Japan Society for Promotion of Science, 43, No. 3 (2002), 257-264 (Literature 1)). Therefore, the industry has been waiting for development of a diffusion barrier layer for effectively preventing diffusion of Al in the aluminum reservoir layer into the substrate.
Heretofore, various diffusion barrier layers for preventing diffusion between a substrate and a coating layer have been proposed (see, for example, S. Govindarajan, J. J. Moore, J. Disam, and C. Suryanarayana; Development of a Diffusion Barrier Layer for Silicon and Carbon in Molybdenum-a Physical Vapor Deposition Approach-, Metallurgical and Materials Transactions A, 30A (1999), 799-806 (Literature 2), M. Takahashi, Y. Ito and M. Miyazaki; Thermal Barrier Coatings Design for Gas Turbines, Proceedings of ITSC' 95, Kobe, (1995), 83-88 (Literature 3), R. A. Page and G. R. Leverant; Inhibition of Interdiffusion From MCrAlY Overlay Coatings by Application of a Ni—Re Interlayer, J. of Engineering for Gas Turbines and Power, 121, (1999), 313-319 (Literature 4), H. Hosoda, T. Kingetsu, and S. Hanada: DESIGN OF OXIDATION RESISTANT COATINGS BASED IN IrAl ALLOY, Proceedings of the 3rd Pacific Rim Conference on Advanced Materials and Processing, edited by M. A. Imam, R. DeNale, S. Hanada, Z. Zhong, and D. N. Lee, TMS (1998), 2379-2384 (Literature 5)). More specifically, used as the diffusion barrier layer are Si or C in Literature 2, Al2O3 in Literature 3, Ni—Re alloy in Literature 4, and Ir in Literature 5, respectively.
In addition, alloy films containing Re-contained alloys as diffusion barrier layers have been proposed (see, for example, T. Narita, M. Shoji, Y. Hisamatsu, D. Yoshida, M. Fukumoto, and S. Hayashi; Rhenium coating as a diffusion barrier on a nickel-based superalloy at high temperature oxidation, MATERIALS AT HIGH TEMPERATURES, 18 (S), (2001), 245-251 (Literature 6), T. Narita, S. Hayashi, M. Shoji, Y. Hisamatsu, D. Yoshida, and M. Fukumoto, Application of Rhenium Coating as a Diffusion Barrier to Improve the High Temperature Oxidation Resistance of Nickel-based Superalloy, Corrosion 2001 NACE International, Houston Tex. (2001), paper 01157 (Literature 7), M. Fukumoto, Y. Matsumura, S. Hayashi, T. Narita, K. Sakamoto, A. Kasama, and R. Tanaka; Coating Formation on the Nb-based Alloys using Electrolytic Process and its Oxidation Behavior, Report of the 123rd Committee on Heat Resisting Materials and Alloys, Japan Society for Promotion of Science, 43, Nos. 3 (2002), 383-390 (Literature 8), T. Narita, M. Fukumoto, Y. Matsumura, S. Hayashi, A. Kasama, I. Iwanaga, and R. Tanaka; Development of Re-Based Diffusion Barrier Coatings on Nb-Based Alloys for High Temperature Applications, MOBIUM for High Temperature Applications, edited by Y-Won Kim and T. Cameiro, TMS (2004), pp. 99-112 (Literature 9), Y. Matsumura, M. Fukumoto, S. Hayashi, A. Kasama, I. Iwanaga, R. Tanaka, and T. Narita, Oxidation Behavior of a Re-Based Diffusion-Barrier/β-NiAl Coating on Nb-5Mo-15W at High Temperatures, Oxidation of Metals, 61, Nos. 1/2, (2004), 105-124 (Literature 10), Y. Katsumata, T. Yoshioka, K. Zaini Thosin, S. Hayashi and T. Narita; Effect of diffusion barrier coating on oxidation behavior of Hastelloy-X at high temperature, Report of the 123rd Committee on Heat Resisting Materials and Alloys, Japan Society for Promotion of Science, 46, Nos. 2 (2005), 183-189 (Literature 11), specification of U.S. Pat. No. 6,306,524 (Literature 12), specification of U.S. Pat. No. 6,746,782 (Literature 13), specification of U.S. Pat. No. 6,830,827 (Literature 14), specification of Japanese Patent No. 3708909 (Literature 15), specification of Japanese Patent No. 3765292 (Literature 16), specification of Japanese Patent No. 3810330 (Literature 17), Japanese Patent Laid-open Publication No. 2001-323332 (Literature 18), Japanese Patent Laid-open Publication No. 2003-213479 (Literature 19), Japanese Patent Laid-open Publication No. 2003-213480 (Literature 20), Japanese Patent Laid-open Publication No. 2003-213481 (Literature 21), Japanese Patent Laid-open Publication No. 2003-213482 (Literature 22), Japanese Patent Laid-open Publication No. 2003-213483 (Literature 23), Japanese Patent Laid-open Publication No. 2004-39315 (Literature 24)). Such alloy films are expected to function as a diffusion barrier layer that prevents mutual diffusion of atoms between a substrate and a coating layer and to thereby solve the above-mentioned problems. For example, in case an aluminum reservoir layer (NiCoCrAlY alloy) is applied as a coating layer onto the surface of a substrate, if a diffusion barrier layer 30 of a Re-contained alloy is interposed between a substrate 10 and an aluminum reservoir layer 20 of NiCoCrAlY alloy as shown in FIG. 1, movement of elements of the aluminum reservoir layer 20 into the substrate 10 and movement of elements of the substrate 10 into the aluminum reservoir layer 20 can be prevented. Formed between the substrate 10 and the diffusion barrier layer 30 is an interlayer 40. This interlayer 40 is involved in the substrate 10. The alloy film shown in FIG. 1 is expected to be capable of maintaining properties of the aluminum reservoir layer 20 and the substrate 10 even under super high temperature environments.
Note that Re—Cr—Ni-based alloys include four phases in their crystal phases, namely, γ phase of a Ni-based alloy containing solid-solved Cr and Re, α phase of a Cr-based alloy containing solid-solved Re and Ni, δ phase of a Re-based alloy containing solid-solved Ni and Cr, and σ phase of a Re3Cr2 alloy containing solid-solved Ni (see W. Huang and Y. A. Chang; A thermodynamic description of the Ni—Al—Cr—Re system, Materials Science and Engineering, A259 (1999), 110-119 (Literature 25)).
Further, it has been reported that the solid-solved amount of Al in the α phase of a Cr—Al system is 0˜45 atomic %, solid-solved amount of Al in the γ phase of a Ni—Al system is 0˜15 atomic %, and solid-solved amount of Al in the δ phase of a Re—Al system is 0˜15 atomic % (see Okamoto H. Journal of Phase Equilibria, (1992) (Literature 26). It has also been reported that, solid-solved amounts of Al and Ni in the α phase of a Ni—Al—Cr system are 0˜45 atomic % and 0˜13 atomic %, solid-solved amounts of Al and Cr in the β phase are 30˜58 atomic % and 0˜11 atomic %, solid-solved amounts of Al and Cr in the γ′ phase are 16˜28 atomic % and 0˜7 atomic %, and solid-solved amounts of Al and Cr in the γ phase are 0˜15 atomic % and 0˜47 atomic % (see N. C. Oforka, C. W. Haworth: Scand, J. Metall. vol. 16, 184-188, (1987) (Literature 27)).
The alloy films proposed by Literatures 2 to 5, which contain Si or C, Al2O3, Ni—Re alloys and Ir as a diffusion barrier layer, involve the problem that, under super high temperature environments, they lose the function as the diffusion barrier layer due to decomposition of the diffusion barrier layer by reaction between the substrate and the aluminum reservoir layer and cannot perform the barrier function over a long period of time.
In the alloy films proposed by Literatures 6 through 24, which include Re-contained alloys as their diffusion barrier layers, Al in the aluminum reservoir layer diffuses into the substrate whereas alloy elements diffuse from the substrate to the aluminum reservoir layer under high temperature environments. As a result, the aluminum reservoir layer gradually changes in constituent elements, composition and texture, and therefore cannot maintain its resisting ability to high temperature oxidization over a long period of time. In addition, alloy elements having diffused into the aluminum reservoir layer deteriorate the adhesion of the Al oxide (Al2O3) film and promote exfoliation, thereby rapidly deteriorating the resistance to oxidization of the aluminum reservoir layer.
In addition, under high temperature environments, such diffusion barrier layers change in thickness, composition and texture, and this causes gradual changes of their barrier properties against diffusion of Al into the substrate and diffusion of alloy elements into the aluminum reservoir layer.
Problems with alloy films having Re-contained alloys and Ni aluminide formed on substrates made of Ni-based heat resistant alloys as diffusion barrier layers and aluminum reservoir layers are discussed below in detail taking Literature 11 as a specific example.
A Ni-based group heat-resistant alloy (HASTELLOY-X (“HASTELLOY” is a trademark)) having the nominal composition shown in TABLE 1 (in weight %) was used as the substrate. This substrate was plated with a Ni—Re alloy and Ni. The substrate coated having the plated film as subjected to Cr diffusion coating. Mixed powder (Cr powder+NH4Cl powder+Al2O3 powder) was used as a Cr diffusion coating agent. The substrate having the plated film was buried into the mixed powder, and heated in an argon gas atmosphere. Heating temperature and time were 1280° C. and four hours. As a result of the plating and Cr diffusion coating, a diffusion barrier layer of a Re-contained alloy is formed. Subsequently, Ni is plated on the surface of the substrate now having the diffusion barrier layer, thereby forming a Ni-plated film. Thereafter, Al diffusion coating was conducted. In greater detail, the substrate having the Ni-plated film was immersed into mixed powder (Al powder+NH4Cl powder+Al2O3 powder (1:1:4 in weight percent) in an alumina crucible, and heated in the argon gas atmosphere at 800° C. for 30 minutes. Next annealed was the substrate after the Al diffusion coating. More specifically, the substrate having passed the Al diffusion coating process was heated in an argon gas atmosphere at 1000° C. for four hours. An alloy film thus obtained was composed of a diffusion barrier layer made of a Re-contained alloy and an overlying aluminum reservoir layer made of Ni aluminide. The Ni heat-resistant alloy having this alloy film was subjected to a high temperature oxidization test. Conditions of the oxidization were: in the atmosphere, at 1100° C., and for 100 hours and 400 hours.
TABLE 1NiCrMoFeWCSiCoMnrest23.515.4619.810.160.491.040.990.53
Test specimens after the high temperature oxidization test at 1100° C. for 100 hours and 400 hours as explained above were cut, ground, observed in cross-sectional texture, and measured in concentration of individual elements by an electron-probe micro analyzer (EPMA). FIG. 2 shows an optical microscopic photograph of a cross-sectional texture of the test specimen subjected to the high temperature oxidization test for 100 hours and concentration distributions (concentration distributions along the X-X line of the photograph) of Ni, Al, Cr, Fe, Mo and Re measured by EPMA. Similarly, FIG. 3 shows an optical microscopic photograph of a cross-sectional texture of the test specimen subjected to the high temperature oxidization test for 400 hours and concentration distributions (concentration distributions along the X-X line of the photograph) of Ni, Al, Cr, Fe, Mo and Re measured by EPMA.
The following results were obtained from the data of FIG. 2.
1) The diffusion barrier layer 30 had the composition of 14.5 atomic % Ni, 20.5 atomic % Mo, 26 atomic % Cr, 28.5 atomic % Re, 7 atomic % Fe, and 0.8 atomic % Al.
2) The diffusion barrier layer 30 formed a continuous layer substantially uniform in thickness.
3) The aluminum reservoir layer 20 had a multiphase structure mainly composed of the β phase 24 and including the γ′ phase 26.
4) The interlayer 40 was formed between the substrate 10 made of Ni-group heat-resistant alloys and the diffusion barrier layer 30.
The following results were obtained from the data of FIG. 3.
1) The barrier layer 30 had the composition of 14 atomic % Ni, 28 atomic % Mo, 26.5 atomic % Cr, 21.5 atomic % Re, 7 atomic % Fe, and 2.5 atomic % Al.
2) The diffusion barrier layer 30 was not uniform in thickness. The layer was lost in the region labeled with numeral 88 and changed to the γ phase.
3) The aluminum reservoir layer 20 had a multiphase structure including the β phase 24 and the γ phase 22, and the γ phase 22 had a multiphase structure existing between the β phase 24 and the diffusion barrier layer 30 and also between the β phase 24 and an alumina scale 50, respectively.
4) The interlayer 40 was formed between the substrate 10 made of Ni-group heat-resistant alloys and the diffusion barrier layer 30, and a concentration gradient of Al and alloy elements was observed.
As shown in FIG. 2, the aluminum reservoir layer 20 contained approximately 30 atomic % Al in average, and it is apparent that the diffusion and invasion of Al into the substrate 10 was limited to small regions. It is further apparent that alloy elements contained in the aluminum r reservoir layer 20 occupied several atomic %. That is, the diffusion barrier layer 30 composed of the Re-contained alloy maintained an excellent diffusion barrier property.
As shown in FIG. 3, as a result of a progress of exhaust of Al in the aluminum reservoir layer 20 due to consumption of Al for formation of the alumina scales 50 and diffusion of Al into the substrate 10 when passing through the diffusion barrier layer 30, as well as a progress of diffusion of alloy elements into the aluminum reservoir layer 20, a part of the aluminum reservoir layer 20 changed from the β phase 24 to the γ phase 22. This γ phase 22 contained a large amount of Mo, Cr, Fe having diffused from the substrate 10.
Further, as shown in FIG. 3 with a channel 88, a part of the diffusion barrier layer 30 disappeared, and changed to a discontinuous layer, with the Re-contained alloy having changed to the γ phase.
From the above review, in some of existing alloy films, decomposition of the diffusion barrier layer 30 progresses under high temperature environments, while unevenly reducing in thickness, and ultimately disappear locally until the substrate 10 (including the interlayer 40) and the aluminum reservoir layer 20 get in direct contact. Differences of texture and form between the diffusion barrier layer 30 of FIG. 2 and that of FIG. 3 are considered partly caused by the difference of oxidation time, but mechanisms of changing texture and structure of existing alloy films and mechanisms of losing the barrier functions still remain unexplained.
Therefore, it is desirable that the diffusion barrier layer 30 can effectively maintain the function to prohibit diffusion of Al in the aluminum reservoir layer 20 into the substrate 10 and diffusion of alloy elements in the substrate 10 into the aluminum reservoir layer 20, and can thereby maintain its texture, composition and structure for a long period of time.